1. Field of the Invention
The present invention relates to .alpha.-alumina and a method for producing the same.
2. Description of the Related Art
Alpha-alumina is widely used as an abrasive, a raw material of a sintered body or a plasma spray material, and a filler. One of the most commonly used economical methods for producing .alpha.-alumina in an industrial scale is a Bayer process. By the Bayer process, bauxite as a raw material is treated with sodium hydroxide to obtain aluminum hydroxide, and then aluminum hydroxide is calcined in an air to obtain .alpha.-alumina.
However, aluminum hydroxide obtained by the Bayer process contains a considerable amount of sodium, and almost all sodium remains in .alpha.-alumina which is obtained by calcining aluminum hydroxide in the air. When .alpha.-alumina is used as an electrical insulating material in a spark plug and the like, the remaining sodium components will interfere the electrical insulating properties.
To solve such problem, some proposals have been made. For example, JP-A-47-5744 discloses a method for removing sodium by wetting aluminum hydroxide with hydrochloric acid or an aqueous solution of aluminum chloride when aluminum hydroxide which is produced by the Bayer process and contains a large amount of sodium is precalcined, and calcining aluminum hydroxide in a furnace with filling wet aluminum hydroxide in a siliceous refractory container.
JP-A-48-34680 discloses a method for calcining aluminum hydroxide or alumina which is obtained by the Bayer process in the presence of hydrochloric acid or aluminum chloride, and boric acid or boron oxide. JP-A-54-16398 discloses a method comprising adding and mixing hydrochloric acid or aluminum chloride and a small amount of a silica-containing material in aluminum hydroxide which is obtained by the Bayer process, calcining the mixture, and then separating alumina from the silica-containing material.
However, an .alpha.-alumina powder produced by the above conventional methods still contain about 400 to 100 ppm of sodium (0.04 to 0.01 wt. % in terms of Na.sub.2 O) even after the treatment for removing sodium, and is not necessarily satisfactory for some applications. Further, in the above methods, it is difficult to remove other impurities such as calcium or iron which will cause abnormal grain growth during sintering of the alumina powder.
In addition, since the .alpha.-alumina particles produced by such methods have irregular shapes and the .alpha.-alumina powder contains many coarse agglomerated particles, they are deagglomerated with a ball mill or a vibration mill to obtain a final product according to the final use. However, deagglomeration of the .alpha.-alumina is not necessarily easy, the deagglomeration is costly, and a long time is consumed for deagglomeration. Therefore, fine particles are generated to widen a particle size distribution, or contamination with impurities decreases a purity of .alpha.-alumina.
U.S. Pat. No. 3,175,883 discloses a method comprising calcining aluminum hydroxide at a temperature of 1000.degree. C. or higher in the presence of a chlorine gas and aluminum trifluoride for removing sodium and controlling the primary particle size of .alpha.-alumina at the same time. A content of sodium in .alpha.-alumina which is obtained by this method is decreased to 30 wt. % or less of the sodium content in aluminum hydroxide as the raw material. But, this method cannot produce .alpha.-alumina containing 100 ppm or less of sodium
U.S. Pat. No. 3,532,460 discloses a method for producing alumina containing a decreased amount of impurities comprising calcining transition alumina which is contaminated with an oxide of gallium, iron, zinc, calcium, manganese, copper or sodium to adjust a water content to 2 to 6 wt. % and a specific surface area in a range between 40 and 120 m.sup.2 /g, and contacting the alumina with a chlorine gas in an oxygen free atmosphere at a temperature of 800 to 1000.degree. C., followed by washing with water and heat treatment. However, the alumina obtained by this method is the transition alumina. While the iron content in the alumina can be greatly decreased by the treatment in the chlorine gas atmosphere, and the contents of sodium and calcium are decreased by the washing with water and heat treatment after the treatment in the chlorine gas atmosphere, this process produces only a transition alumina but cannot produce high purity alumina containing 100 ppm or less of each of sodium and calcium.
JP-A-64-3008 discloses a method for producing easily deagglomerated alumina powder comprising applying mechanical impact on aluminum hydroxide or transition alumina having an average secondary particle size of 10 .mu.m or less which is obtained by the Bayer process, and calcining aluminum hydroxide or transition alumina in the presence of a chlorine-containing compound. However, the disclosed method produces an .alpha.-alumina powder by suspending aluminum hydroxide as the raw material in hydrochloric acid, drying it and then calcining it, but cannot produce high purity .alpha.-alumina containing 100 ppm or less of sodium.